1. Field of the Invention
The present invention generally relates to coatings deposited on components with through-holes that are desired to remain open after the coating process. More particularly, this invention is directed to a method for removing coating deposits from through-holes in a component surface without damaging the hole walls and component surface, and to a gas turbine engine component equipped with cooling holes whose cooling effectiveness is promoted as a result of the removal process.
2. Description of the Related Art
Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. Coating systems capable of satisfying these requirements typically include a metallic bond coat that adheres the thermal-insulating ceramic layer to the component, forming what may be termed a TBC system. Metal oxides, such as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides, have been widely employed as the material for the thermal-insulating ceramic layer. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD). Bond coats are typically formed of an oxidation-resistant diffusion coating such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant alloy such as MCrAlY (where M is iron, cobalt and/or nickel).
While TBC systems provide significant thermal protection to the underlying component substrate, internal cooling of components such as combustor liners and turbine blades (buckets) and nozzles (vanes) is often necessary, and may be employed in combination with or in lieu of a TBC. Air-cooled components of a gas turbine engine typically require that the cooling air flow is discharged through carefully configured cooling holes that distribute a cooling film over the component surface to increase the effectiveness of the cooling flow. The efficiency of a cooling hole can be quantified by the discharge coefficient, Cd, which is the ratio of the effective area of a cooling hole based on flow measurements to the physical area of the hole. The effective area is less than the physical area as a result of surface conditions within the hole, including the entrance and exit of the hole, which provide resistance to air flow through the hole. Consequently, processes by which cooling holes are formed and configured are critical because the size, shape and surface conditions of each opening determine the amount of air flow exiting the opening and affect the overall flow distribution within the cooling circuit containing the hole.
For components that do not require a TBC, cooling holes are typically formed by such conventional drilling techniques as electrical-discharge machining (EDM) and laser machining, or with complex advanced casting practices that yield castings with dimensionally correct openings. Typical discharge coefficients for EDM and laser-drilled cooling holes in air-cooled combustor liners are on the order of about 0.72 and about 0.88, respectively, or less. EDM cannot be used to form cooling holes in a component having a TBC since the ceramic is electrically nonconducting, and laser machining is prone to spalling the brittle ceramic TBC by cracking the interface between the component substrate and the ceramic. Accordingly, cooling holes are often machined by EDM and laser drilling after deposition of the bond coat but prior to application of the TBC. However, the presence of TBC deposits in the cooling holes of an air-cooled component can detrimentally affect the service life of the component as a result of the TBC altering the shape and reducing the size of the cooling hole openings. For TBC's deposited by plasma spraying (APS and LPPS), a significant amount of ceramic can be deposited in the cooling holes when depositing a sufficiently thick TBC to thermally insulate such hot section components as combustor liners. The obstruction of cooling holes with TBC not only occurs with new manufactured air-cooled components, but also occurs when refurbishing a TBC on a component returned from the field. During refurbishing, all of the existing bond coat and TBC are typically removed, and new bond coat and TBC are deposited, with the result that cooling holes can be obstructed by deposits of both the bond coat and TBC materials.
From the above, it can be seen that manufacturing and refurbishing an air-cooled component protected by a TBC is complicated by the requirement that the cooling holes remain appropriately sized and shaped. Typical solutions are to limit the thickness of the TBC applied or, more preferably, perform a final operation to remove ceramic from the cooling holes in order to reestablish the desired size and shape of the openings. Various techniques have been proposed for this purpose. Japanese Laid-Open Pat. No. Heisei 9158702 discloses a process by which a fluid at pressures of 500 kgf/cm2 (about 490 bar) or more is introduced into the interior of an air-cooled component, such that the fluid flows out through the cooling hole openings and, in doing so, removes ceramic material that had blocked the cooling holes as a result of the component being coated with the ceramic material after the cooling hole was formed. Another technique is disclosed in U.S. Pat. No. 6,004,620 to Camm, in which ceramic accumulated in a cooling hole is removed with a jet projected toward the uncoated surface of the hole. Camm uses a jet consisting essential of a liquid, such as water, at very high pressures. Camm teaches that the coating outside of the hole on the coated surface is undamaged because the component itself serves as a mask to prevent the jet from eroding the coating.
While it is known to modify a waterjet to contain an abrasive media (i.e., essentially nonspherical particles with sharp corners and edges), practice has shown that the erosion and abrasion caused by abrasive particles in a water jet at pressures adequate to remove a ceramic deposit can severely damage the cooling hole and the surrounding component surface. In addition, abrasive materials in an abrasive fluid jet fracture to the point where the abrasive media cannot be reused or is difficult to separate from the material removed by the jet. As a result, the spent abrasive fluid must be disposed of, which adds unwanted cost to the process.